Well it depends.
If the gardevoir is 252 sp atk and Speed EV’s with 4 in physical Def, Timid or Modest nature and lands a critical hit moon blast with the timid nature on the 252 sp Def 252 hp Tyrantrum, both without stat buffs or nerfs assuming Pixie Plate, Fairy Aura or any of the numerous berries are in play, a likely 1 or 2 hit KO is in order with moonblast. If said moonblast’s power is decreased by a berry of any source, then the likely 4 atk EV Tyrantrum could respond with a Iron Head. If said Iron head is to gain a Crit or is boosted by an affect like helping hand, then the Gardevor is OHKO’d. If said affects do not take place, then the Gardevor outspeeds the next turn and takes out the Tyrantrum. HOWEVER (you thought I was done, didn’t you) if Lax Incence or Bright powder are held by either Mon, then an Iron Head or Moonblast can miss, causing the other Pokémon to gain an advantage. It is all strictly dependent on how each player builds their respective Pokémon. For curiosity’s sake, however, I ran Pokémon Showdown with my friend, using a blank set Tyrantrum with Iron Head, 4 atk, 252 SpD, and 252 HP EV’s Vs. a Gardevoir 252 Spe, 252 SpA, 4 Def. Neither had held items. It was a single battle with no additional affects. The Gardevor won 3 out of 3 battles.
